1. Field of the Invention
This invention relates to a developing device for developing the electrostatic latent image or the magnetic latent image on an image bearing member into a visible image.
2. Description of the Prior Art
The image formation apparatus with which the present invention is concerned is, for example, an electrostatic copying machine. As is well-known, this machine is such that original information is imaged on a photosensitive drum by optical means to form an electrostatic latent image on the photosensitive medium, where after the latent image is visualized by means of developing powder called toner, the visualized image is transferred to copy paper, and the toner on the copy paper is then fixed to thereby obtain a copy image. There is also known the so-called TES1 system whereby an electrostatic latent image formed on a photosensitive medium is transferred to an insulating member and thereafter the transferred image is developed; or the screen system whereby a secondary latent image is formed by the use of a screen-like photosensitive medium and by ion modulation; or the electrofax system whereby an electrostatic latent image is formed directly on copy paper having a photoconductive layer on its surface and such image is subjected to the developing and fixing treatments to thereby obtain a copy image.
The general developing device is such that a non-magnetic cylinder with toner deposited on the surface thereof (hereinafter referred to as the sleeve) is disposed in proximity to the photosensitive drum and the toner deposited on the sleeve is brought into friction contact with the surface of the photosensitive drum on which the electrostatic latent image rests, to thereby visualize the latent image. In this case, generally, there is a sufficient gap between the photosensitive drum and the sleeve and the gap tolerance does not so much adversely affect the image formation. Accordingly, the developing device is usually placed on and fixed to a developing device mounting member which is usually provided separately. Also, where the toner layer on the sleeve is relatively thin and the gap is small, it is required that the thickness of the toner layer portion brought into friction contact with the drum surface or the width of the nip portion be uniform, and this has heretofore been accomplished by the fixed mounting method as described above.
However, according to such mounting method, there is naturally caused a cumulative error which is attributable to the mechanical dimensional accuracy and in the case of a developing method having a relatively thin toner layer, there is a disadvantage that the dimension of the gap between the drum and the sleeve becomes different from machine to machine due to said cumulative error of each machine and the resultant image, especially the density, becomes irregular.
To eliminate these inconveniences, the applicant has proposed, U.S. patent application Ser. No. 938,101, abandoned, a method in which gap maintaining members are provided at the opposite ends of the sleeve and the sleeve is urged against the photosensitive drum with such gap maintaining members interposed therebetween. According to this method, the gap between the photosensitive drum and the sleeve is maintained always constant by the gap maintaining members, thus eliminating the above-mentioned inconveniences.
Now, in order that the sleeve may be urged against the photosensitive drum with the gap maintaining members interposed therebetween, the following conditions must be satisfied:
(i) The urging pressure must be minimized to the necessary minimum level so that damage and an excessive load are not imparted to the photosensitive drum; and
(ii) When the sleeve is rotatively driven by drive means secured to the photosensitive drum, said urging pressure must not be increased too much or fluctuated by the drive force.